Hi,

Since around 12pm yesterday, all the users are unable to send emails to external addresses.

AM1FFO11FD046.mail.protection.outlook.com
Remote Server returned '550 5.7.1 Service unavailable; Client host [xxx.xxx.xxx.162] blocked using Customer Allow list ; To request removal from this list '

Your message wasn't delivered due to a permission or security issue. It may have been rejected by a moderator, the address may only accept email from certain senders, or another restriction may be preventing delivery.

The following organization rejected your message: AM1FFO11FD046.mail.protection.outlook.com.

 We are using Exchange 2013 and EOP.

Any help would be appreciated.

Kind regards

Vaughan